Spectroscopic analysis generally relies on detection and quantification of emission or absorption of radiation by matter (e.g. individual molecules in analysis of gas phase compounds). The radiation is absorbed or emitted with a particular energy determined by transitions occurring to the molecules of an analyte. For example, in infrared spectroscopy, discrete energy quanta are absorbed by molecules due to excitation of vibrational or rotational transitions of the intra-molecular bonds. The collision of other molecules in a gas mixture with the emitting or absorbing molecules and the collision between the emitting or absorbing molecules themselves can perturb the energy levels of the emitting or absorbing molecules and therefore cause broadening of the emission or absorption line shape. Broadening of spectral line shapes can depend on any or all of the pressure, the temperature, and composition of the gas mixture in addition to the spectral transition and concentration of a particular target analyte. Quantitative measurement errors can occur if the spectroscopic analyzer is used to measure a target analyte in a sample gas with combination of pressure, temperature and composition of background gas that differs from the gas mixture used to calibrate the analyzer. These errors have been found to be a substantial challenge for optical measurement of important trace level impurities (e.g. less than approximately 10,000 ppm) in natural gas quality control, petrochemical production, quality control and environmental emissions control, and the like, but are not limited to those applications. The important impurities can include but are not limited to water (H2O), hydrogen sulfide (H2S), other sulfur compounds, other acids, carbon dioxide (CO2), carbon monoxide (CO), ammonia (NH3), acetylene (C2H2), other hydrocarbons, other hydro-fluoro-chloro-carbons, and combinations thereof.
One or more approaches can be applied to compensate for broadening caused by differences in pressure and temperature during quantitative target analyte analysis. For example, the pressure and/or temperature of the sample gas can be maintained sufficiently close to the calibration gas pressure and/or temperature by proper sample conditioning, including pressure regulation and temperature stabilization of the sample gas. In another example, real time measurement of pressure and temperature can be used to compensate for the collisional broadening change by applying theoretical models, including but not limited to polynomial corrections, pressure temperature matrixes, chemometrics, experimental calibrations, and the like. In another example parameters of the spectroscopic measurement (e.g. the harmonic modulation parameters) can also be adjusted in real time to compensate for line shape broadening due to changes in sample gas pressure. An example of such an approach is described in co-owned U.S. Pat. No. 7,508,521, the disclosure of which is incorporated herein by reference.
Direct absorption spectroscopy approaches can be used for measurement of target analyte concentrations exceeding approximately 10,000 ppm and background gas mixes which offer little or substantially no interfering absorption at the wavelength of the target analyte spectral line. Integration over the some or all of the line shape of the target analyte spectrum can provide a quantitative target analyte concentration, which is proportional to the area of the spectral line shape but does not depend upon the line shape itself.
However, there are currently no available approaches that provide experimental or theoretical compensation of spectral line shape changes caused by collision of the target analyte with molecules in a gas sample having different mass and structure and as a result of changing composition of the gas sample. Compensating for spectral line shape changes caused by changing background sample gas composition is critically important, especially for all harmonic spectroscopy approaches, which typically have to be used to measure target analyte concentrations below approximately 10,000 ppm and from ppb levels (e.g. approximately 1 to 5 ppb) to parts per hundred (e.g. approximately 1% to 10% or even to 75% or higher) in sample gases which include absorption by one or more compounds present at non-negligible concentrations in the background and in applications in which spectrally broadly absorbing gases are present or in which accumulation of condensates on optical surface sin the absorbing beam path is expected to occur. As an example, pipeline corrosion protection and natural gas tariff control in the United States typically require measurement of water vapor (H2O) in natural gas streams within an uncertainty limit of ±4 ppm, over a range of approximately 0 ppm to 400 ppm or higher. The composition of a typical natural gas stream can change over a very wide range, with methane (CH4) tending to vary within a mole fraction range of approximately 50% to 100%; carbon dioxide (CO2) tending to vary within a mole fraction range of approximately 0% to 15%; and ethane (C2H6), propane (C3H8), and butane (C4H10) combined tending to vary in accordance with actual methane and carbon dioxide concentrations to make up 100% of the natural gas stream.
Typical industry standard moisture analyzers based on tunable diode laser spectrometers, for example a SpectraSensors model SS2000 (available from SpectraSensors, Inc. of Houston, Tex.) or a General Electric Aurora (available from GE Measurement & Control Solutions of Billerica, Mass.) may not be capable of providing necessary measurement accuracy over such a wide range of stream component variation due to the spectral line shape broadening caused by unknown gas sample composition. In another example, the U.S. Department of Energy (DOE) sponsored an evaluation project entitled “Development of In Situ Analysis for the Chemical Industry” that was conducted by the DOW Chemical Company and that concluded that harmonic spectroscopy tunable diode lasers are not well suited for gas analysis applications in the chemical industry due to their measurement sensitivity to gas composition changes. The report detailing the results of this study: “In-Situ Sensors for the Chemical Industry—Final Report,” the Dow Chemical Company, Principle investigator: Dr. J. D. Tate, project No. DE-FC36-o21D14428, pp. 1-37, Jun. 30, 2006, is incorporated herein by reference.